# Connecting solar panel and batteries of different voltages in series

Will this work? What I want to do is connect a 12v alternator to a battery to charge it which would then be wired in series with a 24v solar panel, but hopefully isolating the voltage. Will this work? Will this give me ~= 36v that I can use to power my inverter?

It might sort-of work under the right circumstances, but I'm betting you don't have them.

Inverters typically take large currents at the input voltage in order to produce useful power at the higher one. This means that, for your scheme to work, the solar panel must always provide enough current to supply the inverter, ie. be in good sunshine and be capable of suppying the full current required by the inverter while maintaining 16V+.

That's going to be a big panel, and you may have to stop the planet from rotating:)

I don't think your scheme will be successful.

• Yeah, I used my solar panel and it wasn't enough (small 15w panel) and that's why I'm working towards adding a windmill to it and getting more solar panels. I'm probably going to add a second alternator to my bike trainer alternator in parallel with this one as long as this circuit works and won't overpower my battery. Would 24v destroy my 12v battery super quick, or do I have time to set this up and then pull a multimeter on this real quick to check it for overvoltage and unplug if necessary? – mberna May 31 '14 at 13:56
• 15W panel not a chance in hell - it's internal impedance will prevent any chance of the inverter working and you'll be lucky if the panel does not burn out. The only reasonable way of firing up your inverter is directly to a battery of >24V, so gonna be 36. Find some way of charging the battery string with multiple solar panels in series to generate >36V and/or an alternator that can be connected to each 12 V battery in turn so as to charge each one separately. – Martin James May 31 '14 at 15:35

You need a charge controller to limit the charging of a battery. Automobile batteries are not suited to solar power usage. A deep cycle/marine battery is typical in solar systems. Overcharging will damage the battery as will charging at too high a rate. Lead-acid batteries are usually limited to 2A maximum charge rate. Typically, a solar panel(s) feed a charge controller, usually a MMPT type, and the battery is connected directly to an inverter(DC-to-AC). Many, if not most, inverters detect undervoltage and flash an LED. The system should be designed to supply more than enough energy for the tasks required. Know the expected load in kilowatt hours and design accordingly. Bottom line: An auto battery will not last long, especially with an automobile alternator as a charger. An inexpensive PWM charge controller ($20) and an inexpensive auto stereo inverter (300W,$20) plus a marine battery ($120) would be a good system. If well maintained, the typical marine/deep cycle battery will last four or five years under normal use. To get 36VDC as input to your inverter, you'll need batteries setup to deliver 36VDC, such as three 12VDC batteries connected in series. 12VDC+12VDC+12VDC=36VDC. If you're not familiar enough with the technology to use this information posted here, buy a turnkey system. There are batteries designed specifically for solar power that come in different ratings. A 36VDC solar battery with 120Ahr would run about$250. Deep cycle/marine batteries are rated in Amp/hours. A 36VDC/120Ahr battery would run for one hour at 36VDC and with a 120 Amp load.

First off, if you're using an automobile alternator to charge a battery, there's no phase control, i.e. bulk, float, etc. Second, are you using an automobile battery or a deep discharge battery, perhaps a lithium battery? You've not done any research. Auto battery systems are not considered a valid choice for solar PV applications. An alternator is not designed for use with a deep discharge battery. The standard auto battery is considered a bad choice for power storage. You need a deep discharge battery such as a marine or golf cart battery. An alternator cannot provide the 14.5vdc needed for one of the charging phases of a deep discharge battery. In addition to the battery type is the chemistry: lead acid, AGM, lithium-ion. Do a lot of reading. You could end up buying a lot of batteries needlessly ().

• Alternators are used to charge deep-cycle batteries in boats and RVs. A "smart" regulator does do a better job than a standard automotive regulator, however. – Peter Bennett Jul 12 '18 at 22:25

you need a contactor that can switch the battery wires to 12 then back to 24 kinda like the ones on 9 lead dual speed motors or if its 2 batteries 12 v in a series. I would buy 3 transistors not the switch ones but a linear converter LM340K-15 that switches one phase and filter it to 15.0 from 1v to 35 v, i believe theres a 12v of same type or L7812cv at 1.5 amp each so if under 4.5 you can do that. or just a step down module.

• I'd correct your grammar by I don't even understand what you're trying to say. How about adding a schematic for the "3 transistors"? – Sven B Jun 3 '18 at 17:40
• I don't really understand what you're suggesting, but I'm sure it doesn't apply to the OP's situation. Linear voltage regulators won't be of any use to him. – Peter Bennett Jul 12 '18 at 22:27